Solid-state organic light emitting diode (OLED) image display devices are of great interest as a superior flat-panel digital display device. These display devices utilize current passing through thin films of organic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material. Moreover, the display devices can be made with exclusively transparent materials supporting the integration of other optical devices above and below an OLED display device. These organic materials can also be transparent. U.S. Pat. No. 5,776,623 issued Jul. 7, 1998 to Hung et al. entitled Transparent Electron-Injecting Electrode for Use in an Electroluminescent Device describes such a system.
Organic LED materials are also responsive to electromagnetic radiation, and when appropriately biased within an electrical circuit, can produce a current dependent on the ambient light. For example, U.S. Pat. No. 5,929,845 issued Jul. 27, 1999 to Wei et al. entitled Image Scanner and Display Apparatus describes a system which both emits and detects light using an organic electro-luminescent apparatus. Although the transparent organic emissive materials may emit only a limited frequency range of light, other absorptive materials can absorb a very broad spectrum and respond photo-electrically to the presence of light.
The luminance of the organic thin film material also depends upon the age of the material and its prior use. The material degrades over time and through use so that more and more current is required to achieve the same luminance. Moreover, current control in a display device is problematic, especially as compared to voltage control, requiring more circuitry and thus increasing the complexity of the supporting electronics within the display.
Attempts have been made in the prior art to optimize particular display systems to overcome some of the problems noted above. For example, U.S. Pat. No. 5,216,504 issued Jun. 1, 1993 to Webb et al. entitled Automatic Precision Video Monitor Alignment System describes a digital control device within a video monitor to calibrate or otherwise optimize the display, either with human input or under automated computer control.
Some systems integrate user-controlled control mechanisms to provide more flexible operation or optimal use under varying conditions. For example, brightness and contrast controls are often available on video and LCD display devices. These controls can be based on information from the device itself, using a representative pixel within the display. U.S. Pat. No. 5,157,525 issued Oct. 20, 1992 to Eaton et al. entitled Control of Liquid Crystal Display Visual Properties to Compensate for Variation in the Characteristics of the Liquid Crystal describes the use of a representative pixel with separate control to maintain a pre-selected value for contrast or absolute brightness using a feedback arrangement which includes an LCD reference element. The feedback information is determined by measuring the average transmissivity of the LCD material with a photo-detector. U.S. Pat. No. 5,910,792 issued Jun. 8, 1999 to Hansen et al. entitled Method and Apparatus for Brightness Control in a Field Emission Display compares current passing through a resistive layer with that of a current source to provide a feedback compensation for temperature-induced brightness variation. A significant problem with all these approaches is that the feedback does not directly respond to the emissivity of the pixels themselves, or address problems with different types (e.g. colors) of pixels within a display
There is a need therefore for improved detection and control of light from an emissive display.